1. Field of the Invention
The present invention relates generally to a hydraulic flow control system for an automotive vehicle, and more specifically, to a hydraulic flow control system for a power steering unit of an automotive vehicle that can feed an optimum hydraulic flow rate to the power steering unit according to a pump speed or an engine speed so as to satisfy characteristic function of the power steering unit.
2. Description of the Background Art
A hydraulic flow control system for an automotive power steering unit is known in the art, such as, disclosed in Japanese First Patent Publication (Tokkaisho) No. 62-152969, wherein a hydraulic flow rate to be supplied to the power steering unit is adjusted according to a pump speed driven by an automotive vehicle engine. Specifically, the hydraulic flow rate is set larger when an engine speed is lower so as to fully assist a driver to operate a steering wheel smoothly in a low speed range of the vehicle, while, the hydraulic flow rate is set less when the engine speed is higher so as to stiffen the steering response for ensuring the driving safety in a high speed range of the vehicle.
In this publication, a flow control valve unit is provided in a hydraulic circuit of the hydraulic flow control system. The flow control valve unit includes a housing which is formed therein with a blind bore having an open end and a closed end axially spacing from each other. The blind bore is connected to a high pressure side of a hydraulic pump which feeds a pressurized working fluid into the hydraulic circuit, via a high pressure passage formed in the housing, and further is connected to a low pressure side of the pump via a bypass passage which is also formed in the housing. A joining point between the high pressure passage and the blind bore is located closer to the open end of the blind bore than that between the bypass passage and the blind bore.
A union is screwed into the blind bore from its open end to have a first end portion located past the joining point between the high pressure passage and the blind bore. The first end portion is formed in a shape of a truncated cone which is tapered toward the closed end of the blind bore. A throttle passage is defined between the peripheral wall of the blind bore and the tapered peripheral wall of the first end portion to throttle the pressurized working fluid fed through the high pressure passage for increasing the speed thereof. A spool valve is slidably disposed in the blind bore to define a first chamber between the tapered first end of the union and a working end of the spool valve and a second chamber between the other working end of the spool valve and the closed end of the blind bore. A spring is disposed in the second chamber for urging the spool valve toward a position where the spool valve fully closes the bypass passage relative to the first chamber. The throttle passage is opened into the first chamber for generating a hydraulic pressure therein.
The union is formed therein with an axial blind bore which is opened toward a second end of the union located awary from the tapered first end portion. The union is further formed with orifices at the tapered first end portion. Each orifice opens into the throttle passage at its one end and into the axial blind bore of the union at its other end for conducting a controlled flow rate of the working fluid into the axial blind bore of the union, which is then conducted toward a hydraulically operated automotive component, such as, a power steering unit. The union is further formed with a lateral passage which connects the axial blind bore of the union to the second chamber of the blind bore through a communication passage formed in the housing for introducing a hydraulic pressure generated downstream of the orifices into the second chamber.
The operation of the above-noted background art is as follows:
While a pump speed is within a predetermined low range, since a fluid discharge rate of the pump is small enough to provide a less pressure differential between the first and second chambers than a spring force of the spring applied to the spool valve, the spool valve is retained in the position to fully close the bypass passage relative to the first chamber. Accordingly, all the working fluid supplied through the high pressure passage is fed to the hydraulically operated automotive component through the orifices and the blind bore of the union, and its flow rate increases corresponding to increasing speed of the pump.
On the other hand, when the pump speed exceeds the predetermined low range, a flow speed of the working fluid passing through the throttle passage increases to reduce a static pressure thereof such that the flow rate of the working fluid introduced into the blind bore of the union through the orifices is reduced, while a pressure in the first chamber is increased, to generate a pressure differential between the first and second chambers large enough to overcome the spring force applied to the spool valve. Accordingly, the spool valve displaces to a new balanced position to open the bypass passage relative to the first chamber so as to drain a portion of the working fluid to a fluid reservoir, i.e. to a low pressure side of the pump. In general, a flow rate of the working fluid to be drained gets larger corresponding to increasing speed of the pump.
In the above-noted background art, however, since each of the orifices is opened into the throttle passage, when the pump speed increases to exceed a predetermined high level, the static pressure of the working fluid passing through the throttle passage gets so small that a pressure differential between upstream and downstream of each orifice becomes extremely small or a static pressure upstream of each orifice gets smaller than that downstream thereof, thus resulting in that substantially no working fluid is introduced into the hydraulically operated automotive component through the blind bore of the union.